A Dual-Channel Automated Comparator for AC-DC Difference Measurements

An automated ac-dc difference calibration system is described. The system incorporates a new electronic comparator which determines ac-dc differences of thermal voltage converters (TVC´s) by simultaneously measuring the difference between nearly equal ac and dc voltages with both the test and standard instruments. The comparator consists essentially of two practically identical channels each containing a digital-to-analog converter balancing circuit, an operational amplifier to amplify voltage imbalance, and an integrator circuit in which a capacitor is charged during an accurately controlled 10-s period. The difference between the ac and dc voltages applied to the test and standard TVC´s is computed from differences in capacitor voltages, and the ac-dc difference is derived from the variation between the test and standard channel indications of this voltage difference. Measurements are made in about half the time required for our manual procedures.